1. Field of the Invention
The invention relates to an exhaust gas control apparatus and an exhaust gas control method for an internal combustion engine. More particularly, the invention relates to a technology for efficiently using energy of the exhaust gas for purifying the exhaust gas.
2. Description of the Related Art
Generally, in an internal combustion engine mounted in an automobile or the like, especially, in a diesel engine, particulate matter (hereinafter, referred to as “PM”) such as soot in addition to nitrogen oxide (hereinafter, referred to as “NOx”) contained in exhaust gas is required to be removed. In order to satisfy such a requirement, a method is proposed, in which a particulate filter (hereinafter, referred to as a “filter” where appropriate) supporting a storage reduction type NOx catalyst (hereinafter, referred to as a “NOx catalyst” where appropriate) is provided in an exhaust passage of the internal combustion engine.
The NOx catalyst stores NOx in exhaust gas when an oxygen concentration in the exhaust gas flowing therein is high, and releases the stored NOx when the oxygen concentration in the exhaust gas flowing therein becomes low. The filter is formed of a porous base material having multiple pores, and collects PM in the exhaust gas while the exhaust gas passes through the pores. Accordingly, providing the filter supporting the NOx catalyst in the exhaust passage of the internal combustion engine makes it possible to remove NOx and PM contained in the exhaust gas.
If PM is accumulated in the filter, an exhaust path in the filter becomes narrower, and resistance to the exhaust gas flowing through the exhaust path increases. If PM is excessively accumulated in the filter, a pressure of the exhaust gas increases, which causes a decrease in an output from the internal combustion engine. It is therefore necessary to perform a PM recovery process for oxidizing and removing the PM accumulated in the filter so as to recover the PM collecting ability of the filter at the appropriate time.
In the PM recovery process, a temperature of the filter is increased to a value in a high temperature range of approximately 500° C. to 700° C. and an air-fuel ratio of the exhaust gas flowing in the filter is made lean, whereby PM is oxidized and removed.
Fuel used in the internal combustion engine may contain a sulfur (S) component. If such fuel is burned in the internal combustion engine, the sulfur (S) component in the fuel is oxidized to form sulfur oxide (hereinafter, referred to as “SOx”). Therefore, the exhaust gas released from the internal combustion engine contains SOx. If the exhaust gas containing SOx flows in the NOx catalyst, the SOx is stored in the NOx catalyst by the same mechanism as NOx storage. The SOx stored in the NOx catalyst forms stable barium sulfate (BaSO4) with the passage of time. Accordingly, it is difficult to decompose and release the SOx only by decreasing the oxygen concentration in the exhaust gas flowing in the NOx catalyst, and therefore SOx tends to be accumulated in the NOx catalyst.
If an amount of SOx stored in the NOx catalyst increases, the NOx storage ability of the NOx catalyst is decreased, and the ability of removing the NOx in the exhaust gas is decreased. Namely, so-called sulfur poisoning (hereinafter, referred to as “S poisoning”) occurs. Therefore, when the NOx catalyst is provided in the exhaust passage of the internal combustion engine, it is necessary to perform a S recovery process for recovering the NOx catalyst from the S poisoning and recovering the NOx removing ability of the NOx catalyst before the NOx storage ability of the NOx is decreased excessively.
In the S recovery process, an ambient temperature of the NOx catalyst is increased to a value in a high temperature range of approximately 500° C. to 700° C., and fuel serving as a reducing agent is added to the exhaust gas flowing upstream of the NOx catalyst and therefore an air-fuel ratio of the exhaust gas flowing in the NOx catalyst is made rich, whereby SOx is released and reduced.
As described above, in the PM recovery process or the S recovery process, the temperature of the filter or the NOx catalyst needs to be increased to a value in the high temperature range of approximately 500° C. to 700° C. Therefore, the temperature of the exhaust gas released from the internal combustion engine and flowing in the NOx catalyst may be increased to a value in a temperature range in which the catalyst is activated, by subsidiarily injecting fuel in addition to performing main fuel injection.
However, in the internal combustion engine including a centrifugal supercharger, even when the temperature of the exhaust gas released from the internal combustion engine is increased, the energy of the exhaust gas is used for increasing a rotational speed of a turbine. Accordingly, the temperature of the exhaust gas flowing in the NOx catalyst cannot be increased sufficiently.
Also, as the energy of the exhaust gas is used for increasing the rotational speed of the turbine and therefore the rotational speed of the turbine increases, a rotational speed of a compressor also increases and an amount of air taken in a cylinder increases. Accordingly, the intake air amount needs to be adjusted by decreasing an opening amount of an intake throttle valve. As a result, a pumping loss of the internal combustion engine increases, which causes deterioration of fuel efficiency.
In order to address this problem, a technology is proposed, in which a variable nozzle provided in the centrifugal supercharger or a wastegate valve is fully opened such that the energy of the exhaust gas is prevented from being used for increasing the rotational speed of the turbine (refer to Japanese Patent Application Publication No. JP-A-2002-276340, for example).
In the technology disclosed in Japanese Patent Application Publication No. JP-A-2002-276340, since the variable nozzle provided in the centrifugal supercharger or the wastegate valve is fully opened, an amount of energy of the exhaust gas, which is used for increasing the rotational speed of the turbine, decreases. As a result, the intake air amount becomes smaller than that before the variable nozzle or the wastegate valve is fully opened, which may cause an increase in an amount of smoke.